The present invention relates generally to a video signal processing system for processing a wide bandwidth video signal into a reduced bandwidth signal suitable for transmission and/or recording via a narrow bandwidth signal medium whereby the information content of the wide bandwidth video signal is retained in the reduced bandwidth signal and the reduced bandwidth signal is compatible with conventional narrow bandwidth reception apparatus, and for receiving and/or reproducing and processing the transmitted reduced bandwidth signal for recovering therefrom the information content of the original wide bandwidth signal.
The present invention relates more particularly to a signal processing system applicable to a narrow bandwidth format video cassette recorder (VCR) for converting a wide bandwidth input video signal to a reduced bandwidth video signal containing the information content of the input wide bandwidth video signal within the reduced bandwidth whereby the reduced bandwidth video signal may be recorded and reproduced conventionally by such narrow bandwidth format VCR, and for processing the reproduced narrow bandwidth video signal to recover the information content of the wide bandwidth video signal therefrom whereby a wide bandwidth video signal may be reconstructed for yielding improved video bandwidth of the reproduced signal comparable to the full bandwidth of the input video signal, while maintaining backward compatibility of the recorded reduced bandwidth video signal for playing back video cassettes recorded by this improved video signal processing system on available conventional narrow bandwidth format VCRs.
Conventional consumer type VCRs record video information onto video tape cassettes in one of several formats. The well-known VHS format system uses a relatively narrow bandwidth format and produces degraded picture quality in comparison to standard broadcast video chiefly because the recorded VHS format video signal has insufficient horizontal resolution. An enhanced VHS format type recording system, popularly called Super VHS or S-VHS, produces enhanced picture quality by recording a wider bandwidth video signal on the video tape cassette using a higher FM carrier frequency for the luminance information, thus yielding improved picture resolution. Such a format requires a higher FM carrier frequency, higher quality tape in the cassette and higher quality recording and playback mechanisms, heads and circuitry. However, the S-VHS format is not backward compatible with standard VHS format VCRs. That is, although an S-VHS format VCR can reproduce (play back) cassettes recorded on either S-VHS format or standard VHS format VCRs, a standard VHS format VCR cannot play back cassettes recorded on S-VHS format VCRs.
It has long been a goal of video engineers to increase the amount of information transmitted through a given narrowband channel, such as an NTSC signal channel, which is limited to a nominal 4.2 Mhz of useful bandwidth. Because the frame and line rates (temporal and vertical resolution) are usually fixed, restricting the bandwidth translates into restricting the horizontal resolution. In some cases, the nominal bandwidth of the channel is limited to 3 MHz or even 2.5 MHz, resulting in an image with insufficient horizontal resolution.
It has long been recognized that in scanned television systems, the signal energy is concentrated spectrally in the spatio-temporal domain at periodic intervals according to the scanning frequencies, and the video spectrum has so-called `holes`, that is, gaps between these discrete signal areas in which the signal energy is very small, such gaps also occurring at regular intervals. The NTSC composite (i.e. `colorplexed`) color video system represents a system which uses one of these `holes` to carry the color information. In the NTSC system, the signal, i.e., chrominance or `chroma` signal containing the color information is transmitted encoded onto the baseband video as color difference or mixture signal quadrature, i.e. two-phase, amplitude modulation of a suppressed nominally 3.58 MHz sub-carrier (i.e. AM sidebands of a pair of suppressed subcarriers in phase quadrature) which carrier's frequency (3.579545 MHz) is very carefully selected (227.5 times the horizontal scanning frequency of 15.734 KHz) so that a minimum disturbance occurs when a color video signal is displayed on a black and white receiver. Specifically, the NTSC color subcarrier frequency is interleaved horizontally, vertically, and temporally with the luminance signal spectrum to minimize crosstalk and intermodulation between the luminance and chrominance components of the composite video signal.
It was recognized at around the time of the adoption of the NTSC colorplexed system that such frequency spectrum `holes` could also be used to transmit additional horizontal information to increase the horizontal resolution of the reproduced image. In such systems, the higher frequency horizontal information is interleaved spectrally with the lower frequency horizontal information in a similar manner as the chrominance information is in the NTSC color system. An article entitled "REDUCTION OF TELEVISION BANDWIDTH BY FREQUENCY INTERLACE" by E. A. Howson et al. in J. Brit. I. R. E., February, 1960, pp. 127-136 contains a description of such a system which utilized analog signal processing techniques. This described system, however, could not accurately reproduce the full bandwidth image in its original form because it was unable to completely remove the artifacts resulting from the frequency interleaving, which manifested themselves as annoying dot crawl patterns.
Sampled data digital video signal processing techniques were later developed using sub-Nyquist sampling (sometimes termed subsampling) to address the problem. These techniques involved replacing every odd sample in a first video line with a zero-valued sample, and then on the next line, replacing every even sample with a zero-valued sample. On alternate frames, the patterns are reversed.
German Patent Publication No. 82100286.2 entitled "Verfahren zum Ubertagen von Fernsehsignalen uber einen genormten bandbreitebegrenzten Ubertragunskanal und Anordnung zum Durchfuhren des Verfahrens," Jan. 1, 1982 by Professor Wendland et al. describes principles of offset subsampling and bandwidth compression as applied to advanced television systems. This patent publication also describes techniques for implementing television systems in accordance with the described principles.
Theoretically, the Howson et al. frequency folding technique and the sub-Nyquist sampling technique are equivalent, when the folding carrier frequency `f.sub.f ` is one-half the sampling frequency `f.sub.S `. But, although theoretically equivalent, the later sampled data digital systems provided improved reconstruction of the received image because of the existence of line and frame combing techniques, which had not been developed at the time of the Howson et al. system. The sub-Nyquist sampling techniques, however, were developed for totally sampled data digital systems as data reduction (i.e. compression) techniques in digital systems, and the signals generated by these sampling techniques were not generally intended to be passed through a narrowband analog channel.
In an article entitled "DEVELOPMENT OF HDTV RECEIVING EQUIPMENT BASED ON BAND COMPRESSION TECHNIQUE (MUSE)", by Kojima et al. in IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 4, November 1986, pp. 759-768, another data compression scheme is described which achieves bandwidth compression by sampling each pixel once every other frame. This scheme works well for non-moving images. For moving images, a motion vector is developed, and the actual rate of sampling of each pixel is adaptively varied in response to the motion vector so that a sample of the pixel is transmitted every other frame on the average, but more often when that pixel is representing a moving image.
U.S. Pat. No. 4,831,463 issued May 16, 1989 to Faroudja describes apparatus for processing a video signal having a predetermined bandwidth in order to pass the video information through a limited bandwidth channel, such as magnetic tape. In the apparatus described in this patent, a video signal pre-processor includes a comb filter to produce spectral holes, such as described above, between spectrally active areas in the video signal spectrum. A folding circuit folds the baseband video luminance signal about a predetermined folding frequency selected so that aliases of the baseband luminance signal are placed in the spectral holes previously made in the video signal. A low pass filter then filters the resulting folded video signal so that its bandwidth is about one-half (1/2) the bandwidth of the original video signal. The resulting signal may then be transmitted through the limited bandwidth channel.
The Faroudja '463 patent further describes a post-processor which receives the folded signal from the limited bandwidth channel. The post-processor includes an unfolding circuit which unfolds the received signal about a predetermined unfolding frequency. A comb filter then processes the unfolded signal to remove the alias components resulting from the unfolding process. The signal produced by this comb filter closely approximates the original video signal in terms of the bandwidth and information content.
It is interesting to note that the Howson et al. article discussed two bandwidth reduction techniques for video luminance signals by frequency interlacing or interleaving. In the first technique discussed, the video luminance signal spectrum is divided into two equal half-bands (i.e. band-split at frequency `f`) and the upper half-band (i.e. the high band luminance from frequency `f` to frequency `2f`) is used to modulate a sub-carrier which has its frequency set to be near the upper frequency limit of the normal video band (i.e. `2f`). The lower sideband of the modulator output is selected and combined with the original lower half-band, whereby the resultant frequency-interlaced signal after combination contains all of the original luminance signal information but in one-half the bandwidth of the original signal, thus being suitable for transmission over a reduced bandwidth channel.
In the second technique discussed by Howson et al., instead of dividing the main video luminance signal into two half-bands and modulating the `2f` sub-carrier with the high frequency half-band only, the entire main video (i.e. baseband) luminance signal is used to modulate the `2f` sub-carrier. The lower sideband of the modulator output signal contains the required interleaved signal in correct frequency relationship with the main baseband video signal. If the modulator output is added to the main signal and the resultant added signal is passed through a low-pass filter having its cutoff frequency at approximately one-half the sub-carrier frequency, the low-pass filtered output signal consists of the correct composite reduced bandwidth signal (with the sub-carrier suppressed). Howson et al. teach that this second technique avoids the need for using complementary low-pass and band-pass filters as required by the first technique employing band-splitting, and Howson et al. adopted this second technique in a described experimental apparatus, although the summary abstract appearing in the Howson et al. article somewhat misleadingly implies that the first technique using band-splitting was employed.
The folding/unfolding system described in the Faroudja '463 patent is similar in principle to the second technique described and adopted by Howson et al., in that the folding modulation sub-carrier frequency in the Howson et al. system is selected to be an odd multiple of one-half the line scan frequency while the frequency of the folding heterodyne oscillator/mixer or of the sub-Nyquist sampling clock applied to the multiplier used as the folding modulator in Faroudja '463 is selected to be an odd harmonic of one-half the line scan rate or a harmonic of an odd multiple of the line and frame scan rate, and in both systems the folding modulation is performed on the baseband luminance signal and both systems thus necessarily require low-pass filtering after folding to remove frequencies greater than one-half the folding frequency from the folded signal.
Both the Howson et al. article and the Faroudja '463 patent describe folding systems which, if incorporated into an improved VCR, could not produce cassettes which could be played back on conventional VCRs without introducing unacceptable artifacts into the displayed image, that is, recordings of such a folded video signal would lack `backward playback compatibility` to existing VCRs. This is primarily due to the amplitude of the folded high frequency components present within the spectrum of the low frequency components on the previously recorded cassette. The magnitude of the folded high frequency components is sufficiently high as to introduce intolerable artifacts and degradation (dot crawl, twinkling, line flicker, etc.) into an image display produced from a video signal from which the folded high frequency components were not properly removed.
Reference is also made to an article by T. S. Robson entitled "A Compatible High Fidelity Television Standard for Satellite Broadcasting", appearing in "Tomorrow's Television", SMPTE, (1982) at pages 218-236. This article proposed an extended definition `MAC` component type video signal system including filtering high frequency diagonal information from conventional video signals to make available certain gaps in the signal spectrum, and using a 3-D sampling process to deliberately alias useful high frequency information into these gaps, followed by truncation of the original signal spectrum. A post filter/interpolator would be used to restore the folded energy to its correct high frequency location, thereby recreating the original spectrum. This article also describes the necessity for pre- and post-filtering to avoid aliasing in the displayed image. On a conventional display device without 3-D post filtering, however, the high frequency alias product will be present, resulting in image impairment.
It is desirable that an improved video recording system be able to record wider bandwidth video signals on a standard quality cassette than those recordable by conventional narrow bandwidth VCRs, while still maintaining backward compatibility with conventional narrow bandwidth VCRs, and not require especially high quality magnetic tape or record and playback mechanisms. That is, it is desirable that standard quality narrow bandwidth media video cassettes may be recordable with wider bandwidth, higher frequency video information using the improved system and be able to be compatibly played back by conventional narrow bandwidth VCRs without producing noticeable visual artifacts in the reproduced image, even if the conventional VCR may not be able to reproduce the full bandwidth signal recorded on such a cassette.
Howson et al. were not concerned with backward compatibility of the interleaved signal, but instead suggested including a pre-emphasis filter for boosting the interleaved high frequency components of the folded luminance signal in order to minimize the effects of crosstalk from the low frequency luminance components during the transmission of the folded signal through the channel and sub-carrier interference at the receiver. If a video cassette recorded by a VHS format VCR modified to include the system taught by Howson et al. were played back on a standard VHS format VCR, the interference of the pre-emphasized high frequency components which would not be removed would produce an even more objectionable image than that produced by the Faroudja system.
The Faroudja '463 patent does not include any teaching of compatibility with pre-existing recording media and apparatus, other than a mention that such is desirable and is one object of that patent's invention. There is no teaching whatsoever in the Faroudja '463 patent of any apparatus or process for achieving backward or `downward` compatibility with existing playback apparatus. As described above, a recording made by a system according to the Faroudja '463 patent's teaching is not backward compatible with existing playback apparatus because of the high level of the folded luminance high frequencies in the luminance low frequencies.
Thus, the need has remained for improving the video resolution obtainable by the present limited bandwidth video recording and playback techniques, media and mechanisms in a manner which retains backward playback compatibility with existing VCRs and VCPs.
It is an object of the present invention to provide a video signal processing system by which an input wide bandwidth full resolution video signal may be transmitted or recorded as a limited bandwidth video signal as by broadcast or video recording via a limited bandwidth medium, while still retaining in the limited bandwidth signal essentially the information content of the input wide bandwidth signal in such a form that the limited bandwidth signal may be compatibly received or reproduced by conventional narrow bandwidth apparatus to produce a video image without objectionable artifacts.
It is also an object of the present invention to provide a video signal processing system capable of receiving or reproducing such a transmitted or recorded limited bandwidth video signal and reconstructing therefrom a wide bandwidth video output signal corresponding to and containing the information of the input wide bandwidth video signal so as to produce a full resolution video image therefrom.
It is further an object of the present invention to provide a video signal processing system applicable to conventional narrow bandwidth video recording and playback apparatus for improving the resolution of video recordings made and reproduced thereby without requiring higher quality recording and playback mechanisms or recording media.
Still further objects of the present invention include providing a video recording apparatus capable of recording an input wide bandwidth video signal within a limited bandwidth such that the recorded limited bandwidth signal may be compatibly reproducible by conventional narrow bandwidth playback apparatus, and providing a video playback apparatus capable of reproducing such a recorded limited bandwidth signal to recover therefrom a wide bandwidth playback signal corresponding to the input wide bandwidth video signal.
Additional objects of the present invention include providing video signal processing techniques for encoding an input wide bandwidth signal into a limited bandwidth video signal containing in encoded form essentially the information of the input wide bandwidth video signal within a substantially reduced bandwidth and compatibly reproducible by conventional narrow bandwidth playback apparatus for producing a video image without objectionable artifacts, and for decoding such a limited bandwidth video signal to produce therefrom a wide bandwidth video signal corresponding to the input wide bandwidth video signal for producing a video image having the same resolution as the input wide bandwidth video signal.